You look at it as a social media platform. The people who run bot accounts, have specific goals they want to achieve such as swaying public opinion on a specific topic. They could have been paid by a corporation to do damage control on a negative event that happened.

When they downvote you and upvote their other bots, their content gets higher visibility.

Why, I haven’t been so insulted in my life, being compared to a bot. It’s me.

The instance admin can see votes. If you are noticing a pattern maybe ask them to confirm? There can be delays in federation so you might see them all come in at once even if they occurred quite a bit earlier.

I know your question was "why" but maybe confirming that is that is happening vs a different cause can be the first step.

I don't care about the downvotes. I intentionally view ALL posts in the feed, I don't hide anything just because of low score. The lowest score might be the best!